The present invention relates generally to medical devices and more particularly to balloon catheters with a stent mounted thereon.
Stents have become a relatively common medical device for treating a number of organs, such as the vascular system, colon, biliary tract, urinary tract, esophagus, trachea and the like. Stents are useful in a variety of medical procedures and are often used to treat blockages, occlusions, narrowing ailments and other related problems that restrict flow through a passageway. Stents are also useful in treating other ailments including various types of aneurysms and may be useful in numerous other medical procedures as well.
Although stents are used in many different procedures, one common medical procedure in which stents are used involves implanting an endovascular stent into the vascular system. Stents have been shown to be useful in treating numerous vessels throughout the vascular system, including coronary arteries, peripheral arteries (e.g., carotid, brachial, renal, iliac and femoral), and other vessels. However, the use of stents in coronary arteries has drawn particular attention from the medical community because of the growing number of people suffering from heart problems associated with stenosis (i.e., a narrowing of an arterial lumen). This has lead to an increased demand for medical procedures to treat stenosis of the coronary arteries. In addition, the medical community has adapted many intravascular coronary procedures to other intraluminal disorders. The widespread frequency of heart problems may be due to a number of societal changes, including the tendency of people to exercise less while eating greater quantities of unhealthy foods, in conjunction with the fact that people generally now have longer life spans than previous generations. Stents have become a popular alternative for treating coronary stenosis because stenting procedures are considerably less invasive than other alternatives. Traditionally, stenosis of the coronary arteries has been treated with bypass surgery. In general, bypass surgery involves splitting the chest bone to open the chest cavity and grafting a replacement vessel onto the heart to bypass the blocked, or stenosed, artery. However, coronary bypass surgery is a very invasive procedure that is risky and requires a long recovery time for the patient.
Many different types of stents and stenting procedures are possible. In general, however, stents are typically designed as tubular support structures that may be inserted percutaneously and transluminally through a body passageway. Typically, stents are made from a metallic or other synthetic material with a series of radial openings extending through the support structure of the stent to facilitate compression and expansion of the stent. However, other types of stents are designed to have a fixed diameter and are not generally compressible. Although stents may be made from many types of materials, including non-metallic materials and natural tissues, common examples of metallic materials that may be used to make stents include stainless steel, nitinol, cobalt-chrome alloys, amorphous metals, tantalum, platinum, gold, magnesium and titanium. Typically, stents are implanted within an artery or other passageway by positioning the stent within the lumen to be treated and then expanding the stent from a compressed diameter to an expanded diameter. The ability of the stent to expand from a compressed diameter makes it possible to thread the stent through narrow, tortuous passageways to the area to be treated while the stent is in a relatively small, compressed diameter. Once the stent has been positioned and expanded at the area to be treated, the tubular support structure of the stent contacts and radially supports the inner wall of the passageway. As a result, the implanted stent mechanically prevents the passageway from closing and keeps the passageway open to facilitate fluid flow through the passageway. However, this is only one example of how a stent may be used, and stents may be used for other purposes as well.
Particular stent designs and implantation procedures vary widely. For example, stents are often generally characterized as either balloon-expandable or self-expandable. However, the uses for balloon-expandable and self-expandable stents frequently overlap and procedures related to one type of stent are frequently adapted to other types of stents.
Balloon-expandable stents are frequently used to treat stenosis of the coronary arteries. Usually, balloon-expandable stents are made from ductile materials that plastically deform relatively easily. In the case of stents made from metal, 316L stainless steel which has been annealed is a common choice for this type of stent. One procedure for implanting balloon-expandable stents involves mounting the stent circumferentially on the balloon of a balloon-tipped catheter and threading the catheter through a vessel passageway to the area to be treated. Once the balloon is positioned at the narrowed portion of the vessel to be treated, the balloon is expanded by pumping saline through the catheter to the balloon. The balloon then simultaneously dilates the vessel and radially expands the stent within the dilated portion. The balloon is then deflated and the balloon-tipped catheter is retracted from the passageway. This leaves the expanded stent permanently implanted at the desired location. Ductile metal lends itself to this type of stent since the stent may be compressed by plastic deformation to a small diameter when mounted onto the balloon. When the balloon is later expanded in the vessel, the stent once again plastically deforms to a larger diameter to provide the desired radial support structure. Traditionally, balloon-expandable stents have been more commonly used in coronary vessels than in peripheral vessels because of the deformable nature of these stents. In the case of coronary vessels, however, this is generally not a concern since coronary vessels rarely experience traumas transmitted from external sources. In addition, one advantage of balloon-expandable stents is that the expanded diameter of the stent may be precisely controlled during implantation. This is possible because the pressure applied to the balloon may be controlled by the physician to produce a precise amount of radial expansion and plastic deformation of the stent. In addition, balloons with a repeatable diameter for various pressures are typically used.
Stents may also be used in combination with other components to treat a number of medical conditions. For example, stent-graft assemblies are commonly used in the treatment of aneurysms. As those in the art well know, an aneurysm is an abnormal widening or ballooning of a portion of an artery. Generally, this condition is caused by a weakness in the blood vessel wall. High blood pressure and atherosclerotic disease may also contribute to the formation of aneurysms. Common types of aneurysms include aortic aneurysms, cerebral aneurysms, popliteal artery aneurysms, mesenteric artery aneurysms, and splenic artery aneurysms. However, it is also possible for aneurysms to form in blood vessels throughout the vasculature. If not treated, an aneurysm may eventually rupture, resulting in internal hemorrhaging. In many cases, the internal bleeding may be so massive that a patient might die within minutes of an aneurysm rupture. For example, in the case of aortic aneurysms, the survival rate after a rupture can be as low as 20%.
Traditionally, aneurysms have been treated with surgery. For example, in the case of an abdominal aortic aneurysm, the abdomen is surgically opened and the widened section of the aorta is removed. The remaining ends of the aorta are then surgically reconnected. In certain situations the surgeon may choose to replace the excised section of the aorta with a graft material such as Dacron. In still other situations, the surgeon may put a clip on the blood vessel at the neck of the aneurysm between the aneurysm and the primary passageway of the vessel. The clip then prevents blood flow from the vessel from entering the aneurysm.
An alternative to traditional surgery is endovascular treatment of the blood vessel with a stent-graft. This alternative involves implanting a stent-graft in the blood vessel across the aneurysm using conventional catheter-based placement techniques. The stent-graft treats the aneurysm by sealing the wall of the blood vessel with a generally impermeable graft material. Thus, the aneurysm is sealed off and blood flow is kept within the primary passageway of the blood vessel. Increasingly, treatments using stent-grafts are becoming preferred since the procedure results in less trauma and a faster recuperation.
A drug or other bioactive component may also be added to stents to enhance the function of a stent in certain applications. For example, one problem that has been encountered with common stenting procedures is stenosis or restenosis. Stenosis may occur for a variety of reasons, such as the vessel wall collapsing or the growth of new cellular tissue. For example, stenosis may occur as the result of damage caused to the vessel lining during balloon expansion and vessel dilation. This may cause the intima layers of the vessel to attempt to grow new intimal tissue to repair the damage. The tendency of vessels to regrow new tissue is generally referred to as neointimal hyperplasia. In addition, the synthetic materials that are usually used in stents may also contribute to neointimal hyperplasia. This is caused by the body's tendency to grow new living tissues around and over newly implanted foreign objects. The effect of these responses may result in a narrowing or re-narrowing of the vessel (i.e., stenosis or restenosis). However, stenosis is not completely predictable and may occur either abruptly soon after the stenting procedure due to a collapse in the vessel or may occur slowly over a longer period of time for other reasons. In any event, stenosis may defeat the original purpose of the stenting procedure, which is normally to open a narrowed portion of a vessel and to maintain the patency of the vessel.
One approach that has been offered to address the problem of stenosis has been to coat stents with drugs that are designed to inhibit cellular growth. Although many such drugs are known, common examples of these types of drugs include Paclitaxel, Sirolimus and Everolimus. One technique for combining beneficial drugs with an intraluminal device involves coating the drug directly onto the outer surfaces of the device. Alternatively, various pits or reservoirs may be designed into the intraluminal device to receive the drug. Common coating processes include dipping, spraying or painting the desired drug onto the intraluminal device. However, these are only some of the ways that a drug or other bioactive component may be used with a stent. Various other drugs and techniques may also be possible as well.
The above-described examples are only some of the applications in which stents are used by physicians. Many other applications for stents are known and/or may be developed in the future. Moreover, numerous other components may be added to a stent structure in addition to the graft materials and drug coatings described above.